1. Field of the Invention
This invention, in a method for the production of acrylic acid comprising steps for absorbing with water an acrylic acid-containing gas obtained by the reaction of catalytic gas phase oxidation, removing low boiling substances and high boiling substances, and a step for thermally decomposing an acrylic acid oligomer contained in the high boiling substance-containing solution obtained in the step for removal, relates to a method for the production of acrylic acid which allows to prevent the polymerization more efficiently and enjoys exalted productivity.
2. Description of the Related Art
Acrylic acid is used in coating, textile processing, leather products, and building materials, as well as for copolymers destined to produce acrylic fibers and for emulsions to produce tackiness agents and adhesive agents. The demand for acrylic acid is now increasing. With a view to realizing mass production of acrylic acid by using an inexpensive raw material, therefore, it is common for acrylic acid to be produced by the reaction of catalytic gas phase oxidation of propylene, for example. Since the reaction of catalytic gas phase oxidation gives rise to by-production of low boiling substances and high boiling substances besides yielding acrylic acid, various processes are relied on to separate and remove such by-products and purify acrylic acid.
The official gazette of JP-A-09-157213, for example, discloses a method for producing acrylic acid by introducing a mixed gas obtained by the catalytic gas phase oxidation of propylene, for example, to an acrylic acid absorption column, causing the gas to contact an aqueous absorbing solvent containing acrylic acid, acetic acid, and sparingly water-soluble solvent thereby obtaining an aqueous acrylic acid solution, dehydrating the aqueous acrylic acid solution in an azeotropic separation column and obtaining through the bottom of this column acrylic acid substantially free from acetic acid, water, and sparingly water-soluble solvent, meanwhile expelling through the top of the column a mixture consisting of acetic acid, acrylic acid, water, and sparingly water-soluble solvent by distillation, separating the expelled mixture in a storage tank into an organic phase formed substantially of a solvent and a water phase formed of acrylic acid, acetic acid, a solvent, and water, and circulating the organic phase in the azeotropic separation column.
The solution, the exhaust gas, and the like which emanate from such purification processes possibly contain raw material compounds, produced compounds, and other useful compounds. With the object of exalting the efficiency of production, the practice of putting these materials to circulatory use in the process of production is continuing in use.
The official gazette of JP-A-11-012222, for example, discloses a method for recovering acrylic acid from acrylic acid containing acrylic acid dimer and maleic acid, which is characterized by introducing the acrylic acid containing acrylic acid dimer and maleic acid into an acrylic acid-recovering column, expelling acrylic acid by distillation through the top of the column to recover the acrylic acid, meanwhile introducing the bottom liquid (A) from the acrylic acid-recovering column into a thermal decomposition tank and decomposing the acrylic acid dimer in the bottom liquid (A), and then circulating at least part of the bottom liquid (B) from the thermal decomposition tank to the acrylic acid-recovering column. This method is directed toward effective use of the acrylic acid dimer and maleic acid and, by circulating the acrylic acid produced by the decomposition to an acrylic acid-recovering column, is enabled to obtain acrylic acid as a finished product.
Acrylic acid is an easily polymerizing compound and is liable to generate an acrylic acid polymer during the process for absorption of acrylic acid and the subsequent process for purification. Various purification columns, therefore, have been used to produce acrylic acid while preventing the occurrence of polymerization by adjusting distillation pressure, temperature, feed rates of liquids, and the like.
The official gazette of JP-A-2000-355570, for example, discloses a method for distilling an easily polymerizing compound by the use of a distilling device, specifically a method for preventing an easily polymerizing compound from being polymerized, characterized by supplying a liquid substantially identical in composition with the liquid existing in the periphery of component member disposed in the distilling device to the component member by an introducing means by spray. The concept of spraying a liquid substantially identical in composition with the liquid existing in the periphery of component member throughout the entire surface of the component member has originated in the discovery that an easily polymerizing compound begins to polymerize when it is left stagnating on the surface of a component member inside a distilling device and the subsequent discovery that the polymerization within the distilling device would be effectively prevented by performing distillation while allowing no stagnation of the liquid on the surface of the component member in the distilling device. The term “liquid identical in composition” as used herein embraces a feed liquid, a liquid extracted from the interior of the column, a reflux, and a circulating liquid of bottoms (a purified liquid). This liquid diluted to a low concentration with water, alcohol, azeotropic solvent, or extracting solvent can also be used.
Since acrylic acid is an easily polymerizing compound, however, the process for the absorption of acrylic acid and the subsequent process for purification are liable to form acrylic acid polymers. Various purification columns have been used to produce acrylic acid while preventing the occurrence of polymerization by adjusting distilling pressure, temperature, and amount of feed liquid. The control of these factors is not easy because the pressure and the concentration change simultaneously with a change in temperature. The occurrence of an acrylic acid polymer results in lowering the yield of the product.
In the process for the purification of acrylic acid, not only the acrylic acid polymer but also by-products generated by the reaction of catalytic gas phase oxidation adheres to such devices as the distilling column and the occurrence of this adherence entails such harmful effects as blocking the devices and possibly impairs lasting stable production of acrylic acid.
In the high boiling substance-containing solution resulting from the separation of high boiling substances, the so-called Michael type adduct of acrylic acid exists besides the acrylic acid dimer and forms a cause for degrading the efficiency of raw material for the process of acrylic acid production. When the Michael type adduct accumulates in the process, it inflicts a serious hindrance on the process for purification and the process of production as well and entails elevation of temperature and formation of by-products possibly to the extent of degrading the quality of product. When such compounds are recovered as acrylic acid, the recovery possibly results in degrading the quality of acrylic acid.